1. Field of the Invention
The present invention relates to a light-shielding film arranged in a display device to improve the visual quality of color filters in liquid crystal, plasma, EL, and CRT display devices and the like, a method of producing the same, and a fine metallic compound particle composition used therein.
2. Description of the Related Art
A light-shielding film for use in a display device (hereinafter, a “light-shielding film”) refers to a black edge arranged in a display device such as a liquid crystal display, a plasma display, an EL display, a CRT display, or the like; to a checked or striped black edge (black matrix) around pixels; and to a dotted or linear black pattern for light shielding in a Thin Film Transistor (TFT).
An example of a light-shielding film is a black matrix formed around red, blue and green pixels in a color filter, so as to prevent contrast from being reduced by light leakage.
Another example is a light-shielding film arranged on a TFT so as to prevent deterioration of image quality due to electric current leakage caused by TFT light, in a liquid crystal display element in an active matrix driving system utilizing TFT.
In such light-shielding films, a light shielding property equivalent to an optical density of 2 or more is normally required. With regard to the visual quality of the display device, the color of the light-shielding film is preferably black.
Conventionally, when preparing a light-shielding film having high light-shielding properties, use of metal is a viable option. Such a light-shielding film can be prepared, for example, by: forming a metallic film by vapor deposition or sputtering; applying a photoresist onto the metallic film; exposing the photoresist layer to light via a photomask having a pattern for the light-shielding film and then developing it; etching the exposed metallic film; and, finally, removing the resist layer from the metallic film (see “Color TFT Liquid Crystalline Display” (in Japanese), pp. 218-220 published on Jul. 20, 1996 by Kyoritsu Shuppan).
Since a metallic film is used in this method, a high light-shielding effect can be achieved even if the film is thin; however, the method requires the steps of forming a vacuum film, such as vapor deposition or sputtering, and etching step, which significantly increases costs. In addition, because metallic films have a high refractive index, there is also the disadvantage that display contrast is impaired in bright daylight. While it is possible to remedy this problem by using a low-refractive chrome film (consisting of, for example, two layers, such as one of metallic chrome and one of chrome oxide), this inevitably increases costs yet further. Moreover, the chrome used most frequently in this method is problematic in terms of its highly detrimental impact on the environment.
On the other hand, a technique exists whereby a light-shielding film is formed by using carbon black in order to provide a light-shielding film having a low refractive index (see Japanese Patent Application Laid-Open (JP-A) No. 62-9301). Such a light-shielding film is produced by applying a photosensitive resin composition containing carbon black onto a substrate and drying it thereon, followed by light exposure and development.
However, because the optical density of carbon black per unit coating amount is lower than that of fine metal particles, it is necessary to increase the thickness of the film in order to ensure a high shielding property and optical density. This has the disadvantage that when red, blue and green pixels are formed subsequent to the formation of the light-shielding film, bubbles are generated or it is difficult to form uniform pixels. Further, a black matrix containing fine particles of nickel sulfide formed by using an electroless plating technique is known (see JP-A No. 7-218715). However, a disadvantage of this method is that the resulting fine particles have a small particle diameter (average particle diameter 30 nm or less), such that the color of the particles is not absolute black and it is difficult to achieve a film thickness of 0.5 μm or less. Further, the plating solution used in the production process has a major detrimental impact on the environment.